Tachycardia synchronization delays

ABSTRACT

An implantable medical device (IMD) provides for adaptive timing of the delivery of cardioversion shocks. In particular, the invention IMD provides for an adaptive cardioversion synchronization delay with respect to a cardiac event, such as a sensed P-wave or R-wave. When cardioversion with a first synchronization delay fails to terminate a tachycardia, for example, cardioversion may be attempted again with a second synchronization delay. The IMD may keep track of whether each synchronization delay is effective in terminating the tachycardia, and may employ a historically effective synchronization delay when applying cardioversion therapy to treat a subsequent tachycardia episode.

TECHNICAL FIELD

The invention relates to implantable medical devices, and moreparticularly, to implantable medical devices that treat tachycardia.

BACKGROUND

Tachycardia is an abnormal heart rhythm characterized by rapidactivation of one or more chambers of the heart of a patient.Tachycardia is often qualified by the locus of origin: a tachycardiathat originates in the ventricles of the heart is called a ventriculartachycardia (VT) and a tachycardia that originates in the atria of theheart is called an atrial tachycardia (AT) or a supraventriculartachycardia (SVT). Some VTs, if untreated, may accelerate intoventricular fibrillation (VF), in which the pumping ability of the heartis seriously impaired.

There are many therapies that may be applied to treat tachycardia. Sometachycardias respond well to medication, and others may be treated withsurgery such as radio frequency (RF) ablation. In some patients, VT orAT may respond well to antitachycardia pacing (ATP), in which smallelectric stimulations from an implantable pulse generator (IPG) in animplantable medical device (IMD) disrupt the propagation of electricalsignals that cause the tachycardia.

In some circumstances, however, a tachycardia may fail to terminate inresponse to therapies such as these. Some IMDs may therefore include thecapability of delivering a higher energy cardioversion shock toterminate the tachycardia. Cardioversion is an effective therapy intreating well organized single loop tachycardias. Application of acardioversion shock at a particular moment depolarizes cardiac tissue toprevent re-entry, thereby terminating the tachycardia.

In conventional cardioversion therapy, an IMD delivers a cardioversionshock synchronized to a cardiac event, such as an R-wave thataccompanies a ventricular depolarization. In other words, the IMDdelivers the cardioversion shock at a fixed time in relation to theevent. The IMD may, for example, time the delivery of a cardioversionshock promptly upon detection of an R-wave.

An external device, such as an external defibrillator, likewise may becapable of applying cardioversion therapy. Like implanted devices,external devices may sense cardiac events and may apply cardioversionshocks synchronized to the cardiac events.

SUMMARY

In general, the invention provides for adaptive timing of the deliveryof cardioversion shocks. In particular, the invention provides for anadaptive cardioversion synchronization delay with respect to a cardiacevent, such as a sensed P-wave or R-wave. When cardioversion with afirst synchronization delay fails to terminate a tachycardia,cardioversion is attempted again with a second synchronization delay. Amedical device may keep track of whether each synchronization delay iseffective in terminating the tachycardia.

If a synchronization delay is effective, it is more likely to beemployed again to terminate a subsequent tachycardia episode. Aprocessor in a medical device may select a synchronization delay as afunction of historical effectiveness.

In one embodiment, the invention is directed to a method comprisingapplying a first cardioversion shock to a heart experiencing atachycardia. The first cardioversion shock is applied with a firstsynchronization delay with respect to a first cardiac event. The methodalso includes monitoring whether the first cardioversion shockterminates the tachycardia, and applying a second cardioversion shock tothe heart when the first cardioversion shock fails to terminate thetachycardia. The second cardioversion shock is applied with a secondsynchronization delay with respect to a second cardiac event. A thirdcardioversion shock with a third synchronization delay may be applied ifthe second cardioversion shock fails to terminate the tachycardia. Whentreating subsequent episodes of tachycardia, the method may provide thata synchronization delay may be selected that was historically effectivein treating previous episodes.

In other embodiments, the invention may be directed to acomputer-readable medium comprising instructions for causing aprogrammable processor to carry out the techniques described above.

In a further embodiment, the invention presents a medical devicecomprising sensing circuitry to sense a first cardiac event and a secondcardiac event in a heart experiencing a tachycardia, and cardioversioncircuitry to apply a first cardioversion shock and a secondcardioversion shock to the heart. The device further includes controlcircuitry to apply the first cardioversion shock with a firstsynchronization delay with respect to the first cardiac event, and toapply the second cardioversion shock with a second synchronization delaywith respect to a second cardiac event when the first cardioversionshock fails to terminate the tachycardia. The device may further includeone or more sense electrodes to sense the cardiac events, and one ormore cardioversion electrodes to apply the cardioversion shocks. Thedevice may further include a processor to select synchronization delays.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features andinventive aspects of the invention will be apparent from the descriptionand drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of an atrial and ventricular chamberpacemaker/cardioverter/defibrillator with leads extending to a humanheart.

FIG. 2 is a block diagram of the implantable medical device depicted inFIG. 1.

FIG. 3 is a flow diagram illustrating exemplary techniques for applyingcardioversion shocks with synchronization delays.

FIG. 4 is a flow diagram illustrating an exemplary technique forselecting a synchronization delay for a cardioversion shock.

DETAILED DESCRIPTION

FIG. 1 depicts an exemplary implantable medical device (IMD) 10 that maypractice the techniques of the invention. IMD 10 is configured to applycardioversion shocks to heart 12, and to time delivery of thecardioversion shocks adaptively. In particular, IMD 10 appliescardioversion shocks with a synchronization delay with respect to acardiac event, such as a detected P-wave or R-wave, and may apply theshocks to treat atrial tachycardia (AT) or ventricular tachycardia (VT),or both.

In the example of FIG. 1, IMD 10 is an implantable multi-chamberpacemaker/cardioverter/defibrillator that includes anti-tachycardiapacing (ATP), cardioversion and defibrillation capabilities. Theinvention is not limited to the particular IMD shown in FIG. 1, however,but may be practiced by any number of implantable devices. Thetechniques of the invention may be practiced by a device that pacesand/or shocks a single cardiac chamber or several chambers, that pacesand/or shocks one or more atria or one or more ventricles, and thatpaces in any of several pacing modes. Although it is advantageous if theimplantable device is capable of applying ATP, ATP capability is notnecessary to the invention.

Although the invention will be described in the context of an IMD, thetechniques are not limited to application in implantable medicaldevices. An external medical device such as an external defibrillatormay include the capabilities of detecting tachycardia and applying oneor more cardioversion shocks to terminate the tachycardia. The externaldevice may detect cardiac events such as atrial and ventricularactivations, and may apply cardioversion shocks with a synchronizationdelays with respect to a cardiac event.

IMD 10 includes an implantable pulse generator (IPG) (not shown inFIG. 1) that generates pacing stimuli to administer one or more pacingtherapies to heart 12. Pacing therapies may include ATP therapies orantibradycardia pacing, for example. In the embodiment shown in FIG. 1,pacing stimuli may be applied to the right atrium 14 or the rightventricle 16, or both. IMD 10 also includes circuitry to sense atrialand ventricular activations, including activations that may be generatedduring episodes of AT or VT. Atrial and ventricular bipolar pace/senseelectrode pairs at the distal ends of leads 18 and 20, respectively,carry out the pacing and sensing functions.

In right atrium 14, the distal end of atrial lead 18 includes apace/sense tip electrode 22 and a pace/sense ring electrode 24.Pace/sense electrodes 22 and 24 are employed for atrial pacing,including delivery of atrial ATP therapies, and for sensing of P-wavesindicative of atrial activation. The distal end of atrial lead 18 alsoincludes an elongated coil defibrillation electrode 28 that can delivera cardioversion shock or defibrillation shock to right atrium 14.

Atrial lead 18 may include conductors that electrically coupleelectrodes 22, 24 and 28 to IMD 10. The conductors may be arrangedcoaxially, coradially, in parallel, or in another configuration, and maybe insulated from one another and from the tissue of the patient. Theproximal end of atrial lead 18 may include a bifurcated connector 30that couples the conductors to a connector block 32 on IMD 10.

In right ventricle 16, the distal end of ventricular lead 20 likewisemay include a pace/sense tip electrode 34 and a pace/sense ringelectrode 36. Pace/sense tip electrode 34 is deployed in the apex ofheart 12. Pace/sense electrodes 34 and 36 are employed for ventricularpacing, including delivery of ventricular ATP therapies, and for sensingof R-waves indicative of ventricular activation. The distal end ofventricular lead 20 also includes an elongated coil defibrillationelectrode 40 that can deliver a cardioversion shock or defibrillationshock to right ventricle 16. Cardioversion therapy, which is applied totreat VT, typically involves delivery of less energy to heart 12 thandefibrillation therapy, which is applied to treat VF.

Like atrial lead 18, ventricular lead 20 may include one or moreinsulated conductors that electrically couple electrodes 34, 36 and 40to IMD 10. The proximal end of ventricular lead 20 may include abifurcated connector 42 that couples the conductors to connector block32.

FIG. 1 illustrates deployment of a coronary sinus lead 44. Coronarysinus lead 44 may include one or more insulated conductors. The proximalend of coronary sinus lead 44 may include one or more electrodes, suchas pace/sense electrode 46. Pace/sense electrode 46 may be deployedwithin the great vein 48 of heart 12, and may be used to deliver pacingtherapies, including ATP therapies, to the left side of heart 12. Aconnector 50 at the proximal end of the coronary sinus lead 44 couplesthe conductors in lead 44 to connector block 32. In some embodiments ofthe invention, coronary sinus lead 44 may include an elongated exposedcoil wire defibrillation electrode (not shown) that is capable ofapplying cardioversion or defibrillation therapies.

IMD 10 includes a housing 52 that, in some embodiments of the invention,serves as a “can” electrode. In unipolar operation, IMD 10 may deliveran electrical stimulation to heart 12 via an electrode disposed on oneor more of leads 18, 20 or 44, with housing 52 being a part of thereturn current path. In bipolar operation, by contrast, IMD 10 maydeliver an electrical stimulation to heart 12 via a tip electrode, witha ring electrode providing the principal return current path.

In the embodiment depicted in FIG. 1, IMD 10 delivers pacing stimuli toright atrium 14 and right ventricle 16 via electrodes 22 and 34,respectively, and senses activations via the same electrodes. Theelectrodes sense the electrical activity that accompanies AT or VT. Theelectrodes also deliver one or more ATP therapies to treat AT or VT. Theenergy for pacing pulses generated by the IPG, as well as the energy forcardioversion and defibrillation shocks, comes from a power supply suchas a battery (not shown) inside housing 52.

The invention provides techniques for adaptive timing of the delivery ofcardioversion shocks. In particular, the invention provides for anadaptive cardioversion synchronization delay with respect to a cardiacevent, such as a detected P-wave or R-wave. The techniques of theinvention may be applied to treat AT via elongated atrial coil electrode28, or to treat VT via elongated ventricular coil electrode 40, or both.

With these techniques, IMD 10 may apply a cardioversion therapy that ismore likely to treat an atrial or ventricular tachycardia effectivelyand efficiently. The treatment is more likely to be effective becausethe treatment is more likely to terminate the tachycardia. The treatmentis more likely to be efficient because less energy can be used toterminate the tachycardia. In many forms of tachycardia, timing of thecardioversion therapy is important to success, and a cardioversiontherapy applied with one synchronization delay may be as effective ormore effective at a lower energy level than a cardioversion therapyapplied with a different synchronization delay at a higher energy level.As a result, the effective timing of cardioversion therapy may allowtermination of tachycardias with lower energy levels, thereby conservingbattery power of IMD 10. Techniques for application of cardioversiontherapy with an adaptive synchronization delay will be described in moredetail below.

FIG. 2 is a functional schematic diagram of one embodiment of IMD 10 andillustrates how IMD 10 detects episodes of tachycardia and deliverstherapies, such as ATP and cardioversion, to address the episodes. Thisdiagram is exemplary of the type of device in which various embodimentsof the invention may be embodied, and the invention is not limited tothe particular schematic shown. On the contrary, the invention may bepracticed in a wide variety of devices, including single- andmulti-chamber devices, and implantable devices that do not include ATPcapability.

FIG. 2 includes electrode terminals 22, 24, 28, 34, 36, 40 and 46, whichcorrespond to the electrodes shown in FIG. 1. Electrode 60 correspondsto the uninsulated portion of housing 52 of IMD 10. Electrodes 28, 40and 46 are coupled to high voltage output circuit 62, which includeshigh voltage switches controlled by cardioversion/defibrillation(CV/defib) control logic 64 via control bus 66. Switches disposed withincircuit 62 determine which electrodes are employed and which electrodesare coupled to the positive and negative terminals of a capacitor bank68 during delivery of defibrillation or cardioversion shocks.

Electrodes 22 and 24, located on or in right atrium 14, are coupled to aP-wave amplifier 70. Amplifier 70 may comprise an automatic gaincontrolled amplifier providing an adjustable sensing threshold as afunction of the measured P-wave amplitude. Amplifier 70 generates asignal on P-out line 72 whenever the signal sensed between electrodes 22and 24 exceeds the sensing threshold. The time intervals between signalson P-out line 72 reflect the cycle length of atrial activations, and maybe indicative of whether the patient is experiencing an episode of AT.In particular, short cycle lengths may be indicative of AT.

Electrodes 34 and 36, located in right ventricle 16, are coupled to anR-wave amplifier 74. Amplifier 74 may comprise an automatic gaincontrolled amplifier providing an adjustable sensing threshold as afunction of the measured R-wave amplitude. Amplifier 74 generates asignal on R-out line 76 whenever the signal sensed between electrodes 34and 36 exceeds the sensing threshold of amplifier 74. The time intervalsbetween signals on R-out line 76 reflect the cycle length of ventricularactivations and may be indicative of whether the patient is experiencingan episode of VT.

Signals on P-out line 72 and R-out line 76 may be used to signal cardiacevents. In particular, signals on P-out line 72 and R-out line 76reflect sensed atrial and ventricular activations. As will be describedbelow, IMD 10 uses one or more of these cardiac events to time thedelivery of cardioversion shocks. Signals on P-out line 72 and R-outline 76 may further reflect whether a previously detected tachycardiahas been terminated. In general, longer time intervals between signalson P-out line 72 and R-out line 76 may be indicative of a return to anormal sinus rhythm.

A switch matrix 78 may select electrodes for coupling to a wide bandamplifier 80 for use in digital signal analysis. Selection of electrodesis controlled by microprocessor 82 via data/address bus 84. The signalsfrom the selected electrodes are provided to multiplexer 86, and arethereafter converted to multi-bit digital signals by A/D converter 88.The signals may be stored in random access memory (RAM) 90 under controlof direct memory access (DMA) circuit 92.

Digital signal analysis includes, but is not limited to, a morphologicalanalysis of waveforms sensed by the selected electrodes. Morphologicalanalysis may comprise wavelet analysis, Fourier analysis or similarspectral analysis techniques, but the invention is not limited to thoseanalytical techniques. Microprocessor 82 may employ digital signalanalysis techniques to characterize the digitized signals stored in RAM90 to recognize and classify the patient's heart rhythm or to determinethe morphology of the signals employing any of several signal processingmethodologies.

Signals sensed via electrodes 22, 24, 34 and 36 may be used to determinewhether to administer cardiac pacing, ATP, cardioversion ordefibrillation therapies. Pacer timing/control circuitry 94 receivessignals from P-out line 72 and R-out line 76, and computes varioustiming intervals as a function of the timing of the received signals.Pacer timing/control circuitry 94 also may include programmable digitalcounters that control pacing according to any of several pacing modes.

Pacer output circuitry 96 and 98, which are coupled to electrodes 22,24, 34 and 36, generate pacing and ATP stimuli under the control ofpacer timing/control circuitry 94. The IPG of IMD 10 comprisesmicroprocessor 82, in cooperation with pacer timing/control circuitry 94and pacer output circuitry 96 and 98.

Pacer timing/control circuitry 94 may also compute intervals such as R—Rintervals, P—P intervals, P-R intervals and R-P intervals. Theseintervals may be used to detect the presence of a fast heart rate, whichmay be an indicator of a tachycardia or fibrillation. A fast heart ratemay also be indicative of sinus tachycardia, i.e., a fast heart rate inresponse to a physiological stimulus, such as exercise. Microprocessor82 and pacer timing/control circuitry 94 may cooperate to apply any of anumber of algorithms to discriminate a tachycardia such as VT or AT, forwhich antitachycardia therapy may be indicated, from sinus tachycardia,for which therapy is not indicated. Microprocessor 82 and pacertiming/control circuitry 94 may further cooperate to apply any of anumber of algorithms to discriminate a tachycardia such as VT or AT,which may terminate in response to antitachycardia therapies, from othertachyarrhythmias such as atrial fibrillation and ventricularfibrillation, which generally do not respond to antitachycardiatherapies. The invention may be practiced with any algorithm oralgorithms that detect an atrial or ventricular tachycardia.

When IMD 10 detects an atrial or ventricular tachycardia, microprocessor82 may select an ATP regimen that comprises a plurality of ATP therapiesarranged in a hierarchy. In general, the first ATP therapy in ahierarchy is applied initially. If the first ATP therapy fails toterminate the tachycardia, the second ATP therapy in the hierarchy isapplied, and so on. For each ATP therapy that is applied, microprocessor82 loads parameters such as timing intervals from RAM 90 into pacertiming/control circuitry 94, which controls delivery of the ATP therapy.Microprocessor 82 evaluates the outcome of the ATP therapy, anddetermines whether ATP therapy should be discontinued or whether thenext therapy in the hierarchy ought to be applied.

In some circumstances, a tachycardia may be unresponsive to ATPtherapies. In some of those circumstances, cardioversion may beindicated. Cardioversion therapies, like ATP therapies, may differ fromone another and may be arranged in a hierarchy, with the firstcardioversion therapy in the hierarchy applied first, the secondcardioversion therapy in the hierarchy applied if the first fails, andso on. In general, cardioversion therapies may differ from one anotherby the amount of energy delivered during a cardioversion shock. Inaccordance with the invention, cardioversion therapies may also differfrom one another by the synchronization delay.

A synchronization delay is a delay with respect to a cardiac event suchas a detected P-wave, as reflected by a signal on P-out line 72, or adetected R-wave, as reflected by a signal on R-out line 76. Thesynchronization delay may be negligible, essentially zero. In otherwords, the cardioversion shock may be applied immediately upon detectionof the cardiac event. The synchronization delay may cause thecardioversion shock to be applied at a time interval following detectionof the cardiac event. Furthermore, the synchronization delay may be a“negative delay.” Application of a negative delay causes thecardioversion shock to be applied before detection of an expectedcardiac event. In practice, implementing a negative delay involvesobserving past cardiac events and predicting when a future cardiac eventwill occur.

When a cardioversion or defibrillation pulse is required, microprocessor82 may control the timing, strength and duration of cardioversion anddefibrillation pulses. In response to the detection of atrial orventricular fibrillation or tachycardia requiring a cardioversion pulse,microprocessor 82 activates CV/defib control circuitry 64, whichinitiates charging of capacitor bank 68 via charging circuit 100, underthe control of high voltage charging control line 102. The voltage onthe high voltage capacitors is monitored via VCAP line 104, which ispassed through multiplexer 86, and in response to reaching apredetermined value set by microprocessor 82, results in generation of alogic signal on Cap Full (CF) line 106 to terminate charging. Adefibrillation or cardioversion pulse may be delivered by output circuit62.

FIG. 3 is a flow diagram illustrating techniques for adaptive timing ofthe delivery of cardioversion shocks. It is assumed that IMD 10 hasdetected a tachycardia (110) and that cardioversion is indicated. It ispossible that other therapies have been applied prior to cardioversion,but the other therapies failed.

IMD 10 applies a first cardioversion shock to the heart with a firstsynchronization delay (112). The first synchronization delay is withrespect to a first cardiac event. If the detected tachycardia is VT, forexample, IMD 10 may apply the first cardioversion shock to the heartwith respect to a first detected R-wave. The first synchronization delaymay be negligible, or applied at an interval following the first cardiacevent, or may be a negative delay.

IMD 10 monitors whether the first cardioversion shock terminates thetachycardia (114). If so, the IMD 10 may store a record of the successof the therapy in memory 90 (116). In some circumstances, however, thefirst cardioversion shock may fail to terminate the tachycardia, and arecord of the failure may also be stored in memory 90. IMD 10 may applyadditional cardioversion shocks like the first cardioversion shock, withthe same or greater energy and the same first synchronization delay. Ifthis cardioversion therapy fails, IMD 10 selects a secondsynchronization delay (118) and applies a second cardioversion shockwith a second synchronization delay (120) with respect to a secondcardiac event, e.g., a second detected R-wave.

The second synchronization delay may be an offset of the firstsynchronization delay. For example, the second synchronization delay maybe approximately 100 milliseconds longer than the first synchronizationdelay. The second synchronization delay may also be selected from a setof possible synchronization delays. The selected second delay may be anegative delay.

IMD 10 monitors whether the second cardioversion shock terminates thetachycardia (122). If so, the IMD 10 stores a record of the success ofthe therapy in memory 90 (124). Should the second cardioversion shock,applied with the second synchronization delay, fail to terminate thetachycardia after repeated attempts, IMD 10 may continue applyingtherapies (126). Continuing applying therapies may include, for example,selecting a third synchronization delay and applying one or morecardioversion shocks with the third synchronization delay. Continuingapplying therapies may also include applying cardioversion therapies athigher energy levels.

FIG. 4 is a flow diagram that illustrates selection of a synchronizationdelay as a function of historical performance. For purposes ofillustration, it is assumed that the first tachycardia depicted in FIG.3 has been successfully terminated by cardioversion therapy applied withthe second synchronization delay. It is further assumed that IMD 10detects a second tachycardia (130) and that cardioversion is indicatedonce again.

IMD 10 selects a synchronization delay based upon past historicalperformance. In the past, the first cardioversion shock, applied withthe first synchronization delay, failed to terminate the tachycardia.The second cardioversion shock, however, applied with the secondsynchronization delay, was successful in terminating the tachycardia.Accordingly, upon detection of the second tachycardia (130), IMD 10selects the second synchronization delay (132) and IMD 10 applies athird cardioversion shock to the heart with the second synchronizationdelay (134). In other words, IMD 10 selects a synchronization delay thatis historically effective. IMD 10 does not select the firstsynchronization delay, which was historically ineffective with aprevious episode of tachycardia.

IMD 10 continues to monitor the performance of the cardioversiontherapy, in particular, whether the third cardioversion shock terminatesthe tachycardia (136). If so, the IMD 10 may store a record of thesuccess of the therapy in memory 90 (138). If not, IMD 10 may continueapplying therapies (140) as described above.

After several tachycardia episodes, the records stored in memory mayshow that a first synchronization delay is the most effectivesynchronization delay employed so far, a second synchronization delay isthe second most effective synchronization delay, and so on. Based uponthese records, IMD 10 may generate a hierarchy of synchronizationdelays. When cardioversion therapy is applied, the most effectivesynchronization delay may be employed first, with the second mosteffective synchronization delay applied next, and so on. The historicalperformance of each synchronization delay may determine its position inthe hierarchy. When IMD 10 detects subsequent episodes of tachycardia,IMD 10 may select a synchronization delay according to its position inthe hierarchy, i.e., IMD 10 may select a synchronization delay as afunction of historical performance.

IMD 10 may further monitor the effectiveness of cardioversion with eachsynchronization delay, and may omit a historically effectivesynchronization delay when that synchronization delay is no longer ofbenefit to the patient. A synchronization delay may be deemed no longerbeneficial when application of a cardioversion shock with thatsynchronization delay fails to terminate one or more tachycardias, orwhen application of a cardioversion shock with that synchronizationdelay initiates fibrillation and makes the condition of the patientworse.

In addition to selecting synchronization delays, IMD 10 may selectenergy levels of cardioversion shocks by adjusting the amplitude of theshock or the pulse width, or both. In many forms of tachycardia, successof the cardioversion therapy is a function of the timing of the shock,not merely the quantity of energy delivered. It is possible that, with awell-timed cardioversion shock, a tachycardia may be terminated with areduced quantity of energy. IMD 10 may therefore select asynchronization delay to conserve battery power.

The invention further reduces the number of cardioversion shocks appliedto the patient. As IMD 10 acquires and stores more data pertaining tothe effectiveness of cardioversion shocks with synchronization delays,episodes of tachycardia can be terminated more efficiently and withfewer shocks. Because cardioversion shocks are generally uncomfortable,fewer shocks means less patient discomfort. Moreover, the adaptivetiming of the delivery of cardioversion shocks is automatic, andrequires no intervention by the patient or the physician for thepatient.

A single medical device may apply different synchronization delays forAT and VT. In other words, the invention may be applied independently toAT and VT therapies.

In some medical devices, cardioversion and defibrillation functionsoverlap. As noted above in connection with FIG. 2, a device may use manyof the same components when delivering the cardioversion anddefibrillation shocks. Moreover, it may be difficult in somecircumstances to distinguish a VF from a VT, because range of cyclelengths of VF may overlap the range of cycle lengths of VT.

Accordingly, a device that employs a synchronization delay with acardioversion shock may employ a synchronization delay with adefibrillation shock as well. Fortunately, synchronization delays havean indifferent effect upon the efficacy of defibrillation shocks used totreat VF, but may have a favorable effect when a defibrillation shock isapplied to a VT that resembles a VF. When a defibrillation shock isapplied to a VT that resembles a VF, the defibrillation shock is, ineffect, a cardioversion shock. Accordingly, synchronization delays mayimprove the efficacy of cardioversion therapies while doing no harm todefibrillation therapies.

The techniques of the invention also work in harmony with othertherapies, such as ATP therapies. In a typical IMD, for example,adjustments to the timing of cardioversion shocks have no effect uponthe timing of ATP paces.

The preceding specific embodiments are illustrative of the practice ofthe invention. Various modifications may be made without departing fromthe scope of the claims. For example, synchronization delays have beendescribed with respect to cardiac events such as sensed P-waves andR-waves, but the invention is not limited to selecting synchronizationdelays with respect to these particular cardiac events. Embodiments ofthe invention can include cardiac paces, or P-waves and R-waves evokedin response to pacing, as cardiac events. Furthermore, the inventionencompasses selecting a synchronization delays for ventricularcardioversion therapy with respect to a P-wave, and selecting asynchronization delays for atrial cardioversion therapy with respect toan R-wave.

Furthermore, as noted above, the invention is not limited to applicationin an implantable medical device. An external medical device such as anexternal defibrillator may practice the invention. In addition, thetechniques described above may be embodied as a computer-readable mediumcomprising instructions for a programmable processor such asmicroprocessor 82 or pacer timing/control circuitry 94 shown in FIG. 2.The programmable processor may include one or more individualprocessors, which may act independently or in concert. A“computer-readable medium” includes but is not limited to read-onlymemory, Flash memory and a magnetic or optical storage medium. These andother embodiments are within the scope of the following claims.

1. A method comprising: applying a first cardioversion shock to a heartexperiencing a tachycardia, wherein the first cardioversion shock isapplied with a first synchronization delay with respect to a firstcardiac event; monitoring whether the first cardioversion shockterminates the tachycardia; and applying a second cardioversion shock tothe heart when the first cardioversion shock fails to terminate thetachycardia, wherein the second cardioversion shock is applied with asecond synchronization delay with respect to a second cardiac event,wherein the first synchronization delay is negligible.
 2. The method ofclaim 1, wherein the second synchronization delay is approximately 100milliseconds longer than the first synchronization delay.
 3. The methodof claim 1, wherein the first cardiac event comprises a first detectedR-wave and wherein the second cardiac event comprises a second detectedR-wave.
 4. A method comprising: applying a first cardioversion shock toa heart experience a tachycardia, wherein the first cardioversion shockis applied with a first synchronization delay with respect to a firstcardiac event; monitoring whether the first cardioversion shockterminates the tachycardia; and applying a second cardioversion shock tothe heart when the first cardioversion shock fails to terminate thetachycardia, wherein the second cardioversion shock is applied with asecond synchronization delay with respect to a second cardiac event,wherein the second synchronization delay is a negative synchronizationdelay in which the second cardioversion shock precedes the secondcardiac event.
 5. The method of claim 4, further comprising: monitoringwhether the second cardioversion shock terminates the tachycardia; andapplying a third cardioversion shock to the heart when the secondcardioversion shock fails to terminate the tachycardia, wherein thethird cardioversion shock is applied with a third synchronization delaywith respect to a third cardiac event.
 6. The method of claim 1, whereinthe tachycardia is a first tachycardia, the method further comprising:monitoring whether the second cardioversion shock terminates the firsttachycardia; and applying a third cardioversion shock to the heartexperiencing a second tachycardia, wherein the third cardioversion shockis applied with the second synchronization delay with respect to a thirdcardiac event.
 7. The method of claim 1, wherein the tachycardia is afirst tachycardia, the method further comprising: monitoring whether thesecond cardioversion shock terminates the first tachycardia; selecting athird synchronization delay as a function of whether the secondcardioversion shock terminates the first tachycardia; and applying athird cardioversion shock to the heart experiencing a secondtachycardia, wherein the third cardioversion shock is applied with thethird synchronization delay with respect to a third cardiac event.
 8. Amethod comprising: applying a first cardioversion shock to a heartexperiencing a tachycardia, wherein the first cardioversion shock isapplied with a first synchronization delay with respect to a firstcardiac event; monitoring whether the first cardioversion shockterminates the tachycardia; applying a second cardioversion shock to theheart when the first cardioversion shock fails to terminate thetachycardia, wherein the second cardioversion shock is applied with asecond synchronization delay with respect to a second cardiac event; andfurther comprising applying a third cardioversion shock prior to thefirst cardioversion shock, wherein the third cardioversion shock isapplied with the first synchronization delay with respect to a thirdcardiac event.
 9. The method of claim 8, further comprising: applyingthe first cardioversion shock at a first energy level; and applying thesecond cardioversion shock at a second energy level.
 10. The method ofclaim 9, wherein the second energy level is less than the first energylevel.
 11. The method of claim 8, further comprising storing in memory arecord of whether the first cardioversion shock successfully terminatesthe tachycardia.
 12. The method of claim 11, wherein the record is afirst record, the method further comprising: storing in memory a secondrecord of whether the second cardioversion shock successfully terminatesthe tachycardia; and generating a hierarchy of synchronization delays asa function of the first and second records.
 13. A computer-readablemedium comprising instructions for causing a programmable processor to:apply a first cardioversion shock to a heart experiencing a tachycardia,wherein the first cardioversion shock is applied with a firstsynchronization delay with respect to a first cardiac event; monitorwhether the first cardioversion shock terminates the tachycardia; andapply a second cardioversion shock to the heart when the firstcardioversion shock fails to terminate to terminate the tachycardia,wherein the second cardioversion shock is applied with a secondsynchronization delay with respect to a second cardiac event, whereinthe first synchronization delay is negligible.
 14. The medium of claim13, wherein the second synchronization delay is approximately 100milliseconds longer than the first synchronization delay.
 15. The mediumof claim 13, wherein the first cardiac event comprises a first detectedR-wave and wherein the second cardiac event comprises a second detectedR-wave.
 16. A computer-readable medium comprising instructions forcausing a programmable processor to: apply a first cardioversion shockto a heart experiencing a tachycardia, wherein the first cardioversionshock is applied with a first synchronization delay with respect to afirst cardiac event; monitor whether the first cardioversion shockterminates the tachycardia; and apply a second cardioversion shock tothe heart when the first cardioversion shock fails to terminate thetachycardia, wherein the second cardioversion shock is applied with asecond synchronization delay with respect to a second cardiac event,wherein the second synchronization delay is a negative synchronizationdelay in which the second cardioversion shock precedes the secondcardiac event.
 17. The medium of claim 16, the instructions furthercausing the processor to: monitor whether the second cardioversion shockterminates the tachycardia; and apply a third cardioversion shock to theheart when the second cardioversion shock fails to terminate thetachycardia, wherein the third cardioversion shock is applied with athird synchronization delay with respect to a third cardiac event. 18.The medium of claim 16, wherein the tachycardia is a first tachycardia,the instructions further causing the processor to: monitor whether thesecond cardioversion shock terminates the first tachycardia; and apply athird cardioversion shock to the heart experiencing a secondtachycardia, wherein the third cardioversion shock is applied with thesecond synchronization delay with respect to a third cardiac event. 19.The medium of claim 16, wherein the tachycardia is a first tachycardia,the instructions further causing the processor to: monitor whether thesecond cardioversion shock terminates the first tachycardia; select athird synchronization delay as a function of whether the firstcardioversion shock terminates the first tachycardia and whether thesecond cardioversion shock terminates the first tachycardia; and apply athird cardioversion shock to the heart experiencing a secondtachycardia, wherein the third cardioversion shock is applied with thethird synchronization delay with respect to a third cardiac event.
 20. Acomputer-readable medium comprising instructions for causing aprogrammable processor to: apply a first cardioversion shock to a heartexperiencing a tachycardia, wherein the first cardioversion shock isapplied with a first synchronization delay with respect to a firstcardiac event; monitor whether the first cardioversion shock terminatesthe tachycardia; and apply a second cardioversion shock to the heartwhen the first cardioversion shock fails to terminate the tachycardia,wherein the second cardioversion shock is applied with a secondsynchronization delay with respect to a second cardiac event, theinstructions further causing the processor to apply a thirdcardioversion shock prior to the first cardioversion shock, wherein thethird cardioversion shock is applied with the first synchronizationdelay with respect to a third cardiac event.
 21. The medium of claim 20,wherein the first cardioversion shock is applied at a first energylevel, and wherein the second cardioversion shock is applied at a secondenergy level.
 22. The medium of claim 21, wherein the second energylevel is less than the first energy level.
 23. The medium of claim 20,the instructions further causing the processor to store in memory arecord of whether the first cardioversion shock successfully terminatesthe tachycardia.
 24. The medium of claim 23, wherein the record is afirst record, the instructions further causing the processor to: storein memory a second record of whether the second cardioversion shocksuccessfully terminates the tachycardia; and generate a hierarchy ofsynchronization delays as a function of the first and second records.